Added a UI option to play all nonlands to the stack instead of the battlefield

By default Cockatrice will only play a card onto the stack if it's an instant or sorcery; otherwise, it plays the card straight onto the battlefield. Rules-wise, this is incorrect behavior -- all spells (i.e. nonlands) use the stack.

This change adds an option, which is turned off by default, to play double-clicked (or single-clicked, depending on your settings) nonland cards onto the stack rather than onto the battlefield.